1. Field of the Invention
This invention relates to the field of antistatic compositions suitable for application to polymeric materials, and particularly relates to the application of a thin antistatic composition to the surface of a polyester support for photographic films.
2. Background Art
Due to their high dielectric constant it is a common characteristic of many polymeric articles to build up static charges when moving in contact with surfaces of similarly high dielectric constant. Films, sheets, and rolls used as elements and supports for photographic applications are required to undergo increased relative motion as a result of automation and higher speed production techniques.
While static discharge has been a continuing problem with such insulating structures, the problems associated with photographic films are amplified because highly sensitive recording media (e.g. silver halide emulsions) are coated on the supports. In fact, very high speed photographic emulsions are capable of detecting light generated by static discharge which is not visible to the naked eye or easily detected by simple instrumentation. In addition, where films are used in rolls or reels which must be mechanically wound and unwound and used for duplication, i.e., microfilm or medical recording film used to store and later reproduce information, small dirt particles clinging to the surface due to static attraction will be magnified in enlargements. Thus, because static-related damage may result after, as well as before, the photosensitive element has been exposed and developed, it is desirable to provide antistatic protection which survives such processing.
Several methods are known for protecting photographic materials from the adverse effects of static.
Matting agents or particulates, i.e., beads or spheres in photographic coatings, can physically separate the film from the roller or plate. This approach has the disadvantage of creating haze, and the particulates may become dislodged and cause dust and dirt problems, which is unacceptable for high quality film uses.
Hygroscopic materials can prevent static by overcoming the low moisture conditions conducive to static, but have the undesirable effect of causing sheets or films to stick together to stick to other surfaces.
Electroconductive polymers can be incorporated into a layer to provide static protection by preventing the static buildup through electrical conductivity. A common problem with these is that they are not permanent to photographic processing solutions and some are not transparent when coated in admixture with conventional binders.
U.S. Pat. No. 4,225,665, Schadt, incorporated herein by reference, describes an antistatic layer comprising a mixture of a water-soluble, electrically conductive polymer having functionally attached carboxylic groups integral or incorporated therein, a hydrophobic polymer binder, and a polyfunctional aziridine crosslinking agent. This composition is coated over a resin-subbed (subcoated) polyester support to give antistatic protection thereto. The trouble with this system is that mixing of the crosslinking agent and the other components prior to coating causes untold problems. Premature reactions occur that plug the coating equipment, and thick layers must be applied in order to achieve good antistatic protection. This is a costly system and thicker layers also have other problems such as increased haze.